Recently, based on the wavelength division multiplexing (WDM) scheme which is able to handle a rapid increase in Internet traffic, an optical transport network (OTN) has been recommended by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) as a so-called transparent transmission platform in which an upper layer does not have to be aware of a lower layer when a client signal is transmitted from end to end not only in a synchronous network, such as a synchronous digital hierarchy (SDH) or a synchronous optical network (SONET), but also in an asynchronous network, such as a network based on the Internet Protocol (IP) or the Ethernet (registered trademark). The interface and the frame format of the OTN are standardized in ITU-T Recommendation G. 709, and have been rapidly introduced into commercial systems.
FIG. 1 is a configuration diagram illustrating an exemplary network of a SONET transmission system. As illustrated in FIG. 1, the transmission apparatuses 1A, 1B, 1C, and 1D form a ring network having a redundant configuration including a working line (Work) through which a signal is transmitted in the clockwise direction as indicated by a solid line, and a protection line (Protection) through which a signal is transmitted in the counterclockwise direction as indicated by a dotted line. The transmission apparatuses 1B, 1C, and 1D operate in synchronization with the master clock of the transmission apparatus 1A.
FIG. 2 is a configuration diagram illustrating an exemplary transmission apparatus in a SONET transmission system. In FIG. 2, a signal which is input from the client-side interface is terminated in a SONET-frame synchronizing circuit 2. The clock of the signal is then switched to the system clock generated by a clock generator 4 in a clock-switch stuff generating circuit 3. Then, the signal is mapped into a SONET frame in a SONET-frame generating circuit 5 and is output to a network via the network-side interface.
In the same manner, an input signal from the network-side interface is terminated in a SONET-frame synchronizing circuit 6. The clock of the input signal is then switched to the system clock generated by the clock generator 4 in a clock-switch circuit 7. Then, the signal is mapped into a client frame in a client-frame generating circuit 8 and is output to a client transmission path via the client-side interface.
The network-side interface has a redundant configuration including the working line (Work) and the protection line (Protection) which are used in such a manner that, when a failure occurs in the working line (Work), the data transmission line is switched from the working line to the protection line so as to restore the transmission of a signal.
In the SONET transmission system, a switching control time period in which a line-disconnection alarm is detected when a line disconnection occurs in the working line, and in which the line-disconnection alarm causes switching to be performed between the working line and the protection line is approximately 40 ms or less. This complies with the network system request that the switching time be 50 ms or less.
In the SONET transmission system, a synchronized clock is used in the entire network, and all of the signal processors in an apparatus operate by using the synchronized clock. In contrast, the OTN transmission system is applied to an upper layer of the SONET transmission system, and is regarded as being equivalent to a transmission path in the WDM system. Accordingly, a signal is desirably transmitted transparently from the client interface. The client interface desirably transmits client signals having various transmission rates for, for example, the Ethernet (registered trademark) and a fiber channel in addition to a SONET/SDH interface. Therefore, a client signal and a network signal are asynchronously processed. When a client signal is transmitted to the network signal side, the frequency components of the signal are also transmitted as information, and the receiver side reproduces the client interface signal from the received frequency components.
FIG. 3 is a configuration diagram illustrating an exemplary network of an OTN transmission system. As illustrated in FIG. 3, transmission apparatuses 11A, 11B, 11C, and 11D form a ring network having a redundant configuration including a working line (Work) through which a signal is transmitted in the clockwise direction as indicated by a solid line and a protection line (Protection) through which a signal is transmitted in the counterclockwise direction as indicated by a dotted line. The transmission apparatuses 11A, 11B, 11C, and 11D operate asynchronously.
FIG. 4 is a configuration diagram illustrating an exemplary transmission apparatus of an OTN transmission system. In FIG. 4, a client signal from a client transmission path is converted into an electric signal in an optical/electronic (O/E) converter 21, and the client clock is extracted in a client interface 22. Then, the client signal is supplied to an ODU-frame stuff generating circuit 23. The ODU-frame stuff generating circuit 23 maps the client signal into an ODUk frame. At that time point, justification control (JC) bytes which are stuff information serving as frequency adjustment information of the client signal are added to the overhead of the ODUk frame, and stuff bytes for absorbing the time-base change for the client signal are inserted into the payload area or the overhead area of the ODUk frame.
The ODUk frame that is output from the ODU-frame stuff generating circuit 23 is mapped into an internal frame in an internal-frame stuff generating circuit 24. The internal frame is transmitted through a cross connector and a multiplex separator (which are not illustrated) and is terminated in an internal-frame stuff terminating circuit 25 so as to be output as an ODUk frame. A clock generating circuit 26 generates the system clock and supplies the system clock to, for example, the ODU-frame stuff generating circuit 23, the internal-frame stuff generating circuit 24, and the internal-frame stuff terminating circuit 25.
An overhead and a forward error correction (FEC) are added to the ODUk frame in OTU-frame generating circuits 27A and 28B for the working line and the protection line, respectively, so as to form OTUk frames. The OTUk frames are converted into optical signals in electronic/optical (E/O) converters 28A and 28B, and are transmitted to the OTN network.
An OTU signal from the working line of the OTN network is converted into an electronic signal in an optical/electronic (O/E) converter 31A and is terminated in an OTU-frame synchronizing circuit 32A so as to be output as an ODUk frame and be supplied to a selector (SEL) 35. A clock generating circuit 33A generates a clock that is synchronized with the network clock extracted from the OTU signal, and supplies the generated clock, for example, to the OTU-frame synchronizing circuit 32A and an internal-frame stuff generating circuit 36 described below. Similarly, an OTU signal from the protection line of the OTN network is converted into an electronic signal in an optical/electronic (O/E) converter 31B and is terminated in an OTU-frame synchronizing circuit 32B so as to be output as an ODUk frame and be supplied to the selector (SEL) 35.
An ODUk frame selected by the selector 35 is mapped into an internal frame in an internal-frame stuff generating circuit 36. At that time, stuff for absorbing the time-base change for an OTUk frame in the network is generated and inserted into the internal frame. The internal frame is transmitted through a cross connector and a multiplex separator (which are not illustrated) and is terminated in an internal-frame stuff terminating circuit 37 so as to be output as an ODUk frame.
The ODUk frame is supplied to an ODU-frame stuff terminating circuit 38, in which data, the clock, and a write enable signal are extracted from the ODUk frame and are supplied to a clock switch memory 39. Then, data in the payload area in the ODUk frame, i.e., client signal data, is written into the clock switch memory 39. The system clock generated in the clock generating circuit 26 is supplied to the internal-frame stuff generating circuit 36, the internal-frame stuff terminating circuit 37, the ODU-frame stuff terminating circuit 38, and the clock switch memory 39.
The ODU-frame stuff terminating circuit 38 specifies the positions of inserted stuff bytes on the basis of stuff information, i.e., JC bytes, extracted from the overhead of the ODUk frame, and generates a write enable signal instructing that the overhead area and the stuff bytes are not to be written and that only data parts of the payload area are to be written. Therefore, the write enable signal that is output from the ODU-frame stuff terminating circuit 38 corresponds to the transmission rate, i.e., the stuff information, of a client signal in a transmission apparatus on the transmission side.
In addition, the write enable signal that is output from the ODU-frame stuff terminating circuit 38 is supplied to a phase locked loop (PLL) 40 which serves as a phase synchronizing circuit. The PLL 40 generates a clock which is synchronized with the write enable signal and in which the transmission rate of a client signal is smoothed. The PLL 40 supplies the generated clock as a read clock to the clock switch memory 39 and also to a client transmission interface 41.
Thus, the client signal data is read from the clock switch memory 39 and is output as a client signal from the client transmission interface 41. The client signal is transmitted through a selector (SEL) 42, is converted into an optical signal by an electronic/optical converter 43, and is transmitted to the client transmission path.
An alarm signal that indicates occurrence of a signal disconnection or the like which is detected in the optical/electronic converter 31A or 31B or the OTU-frame synchronizing circuit 32A or 32B for the working or protection line is supplied to a switch controlling circuit (SW CONT) 45 via an OR circuit 34A or 34B. Under the control performed by the switch controlling circuit 45, the selector 35 selects either one of the output signals from the OTU-frame synchronizing circuits 32A and 32B and supplies the selected signal to the internal-frame stuff generating circuit 36, and the selector 42 selects either one of the client signal that is output from the client transmission interface 41 and an alarm indication signal (AIS) generated by an AIS generating circuit 44 and supplies the selected signal to the electronic/optical converter 43.
FIG. 5 illustrates a timing chart of an operation in which switching to the protection line side is performed. A redundant switching operation of an OTN transmission system will be described below. That is, the operations in which, when a line disconnection occurs in the working line in FIG. 4, switching to the protection line is performed will be described with reference to the timing chart in FIG. 5. In FIGS. 4 and 5, the symbol (A) denotes the state of the working line on the network side. The symbol (B) denotes the alarm detection result of the working line on the network side. The symbol (C) denotes a first control signal that is output from the switch controlling circuit 45 in order to select either the working line or the protection line on the network side. The symbol (D) denotes the state of the selected line. The symbol (E) denotes a second control signal that is output from the switch controlling circuit 45 in order to output an AIS that notifies the upper layer of a failure. The symbol (F) denotes an output signal of the client side interface. The symbol (G) denotes the operation of the PLL 40.
In FIG. 5, at time T11, a line disconnection occurs in the working line on the network side, and, at time T12, the OTU-frame synchronizing circuit 32A on the network side detects an alarm indicating the line disconnection. At time T13, the switch controlling circuit 45 switches from the working line to the protection line. At time T14, the PLL 40 completes a synchronous pull-in operation in the clock.
The time period for the redundant switching operation will be described below. The time period for detecting a line-disconnection alarm, i.e., between T11 and T12, is approximately 1 ms or less. The switching control time period, i.e., between T12 and T13, is approximately 40 ms. In the switching control time period, the oscillation frequency of the PLL 40 goes up to the maximum frequency or down to the minimum frequency. In this state, the PLL 40 oscillates at the possible maximum or minimum frequency. Since the PLL 40 starts a synchronous pull-in operation from the maximum or minimum frequency to the frequency of a client signal on the protection line side after the main signal is switched, the synchronous pull-in time (i.e., between T13 and T14) is approximately 3 sec or less. The above-described redundant switching operation takes up to approximately 3 sec.
A technology has been proposed which achieves uninterrupted switching in a data processing apparatus that supports redundant switching (for example, see Japanese Laid-open Patent Publication No. 2010-226200).